1. Field of the Invention
This invention is a further development in the field of acoustical imaging, with particular application to the imaging of anatomical organs.
2. Description of the Prior Art
Acoustical imaging of anatomical organs of the human body has heretofore generally been accomplished by the pulsed-echo technique, whereby an electrical pulse excites an acoustical transducer to launch a compressional wave into the body. As the compressional wave passes from one region of the body to another region having a different acoustical impedance, part of the wave energy is reflected at the interface between the two regions back toward the transducer. The remainder of the wave energy is transmitted deeper into the body tissues until another acoustical impedance discontinuity is reached, whereupon another partial reflection and partial transmission of the wave energy occur. The reflected acoustical signals are converted by the transducer into electrical signals, which are amplified as necessary. These electrical signals can be processed to generate an image of the organ being examined. From a knowledge of the velocity of a compressional wave in the various tissues of the body, it is possible with the pulse-echo technique to measure the depths at which the various reflections occur within the body by relating the times of arrival at the transducer of the reflected signals to the time of the initially emitted pulse.
In the contact scanner system, which is a particular application of the pulse-echo technique, a single transducer is used to launch a parallel beam of ultrasonic wave pulses into the body and to receive any waves that may be reflected from impedance discontinuities. The position and orientation of the ultrasonic beam transducer are determined for the particular organ to be scanned by suitable linkages to position-determining transducers that are coupled to a storage-type oscilloscope. In operation, the ultrasonic beam transducer is moved over the surface of the body, and an image of the internally reflecting surfaces within the body is built up on the screen of the oscilloscope. Typically, a period of approximately 20 seconds is used to move the transducer over the body surface so as to form a suitable image for display and analysis. A significant disadvantage of the contact scanner system is the relatively long time required to form an image. Such a long time is likely to result in loss of resolution due to movement of the patient or involuntary movement of the bodily organ being imaged (e.g., by the beating of the heart).
It was also known to the prior art to use a linear array of acoustical transducers. In such an arrangement, each transducer would be automatically time-multiplexed so that only one transducer at a time would emit a pulse. Since multiplexing can be done much faster than any corresponding mechanical movement of a contact scanner as described above, an image can be formed from an array of 30 transducers in a total of about 30 milliseconds. Such a short time period makes it possible to observe the movements of an internal organ such as a beating heart. A major disadvantage of such a linear array system, however, is that the length of the array must be equal to a linear dimension of the object to be examined. Consequently, with a linear array of transducers, a large ultrasonic aperture in the body is required in order to image an internal organ. Unfortunately, ultrasonic absorption by bone tissue is extremely high in comparison with ultrasonic absorption by soft tissues. Consequently, bone tissue will shadow any soft-tissue structure located behind it. Where the organ to be imaged is located within the rib cage (e.g., the heart), the overlying rib cage presents an obstacle to the imaging of the organ by a linear array of transducers. Furthermore, the inherent divergence of an ultrasonic beam emanating from a linear array of transducers severely limits the resolution obtainable for objects located deep within the body.
In order to circumvent the limitation on resolution inherent in a linear array of transducers, it was known to use an acoustical lens in combination with such a linear array of transducers. The acoustical lens serves to focus the ultrasonic beam from each transducer of the array onto a particular point on a focal surface within the body. The acoustical lens in such prior art imaging systems was physically separated from the transducer array, typically by an intervening water bath. The intervening water bath caused such imaging systems to be heavy and mechanically complex, and thereby effectively precluded the design of a convenient hand-held lens system.
Acoustical imaging systems known to the prior art did not use a homocentric lens, and hence were troubled by off-axis aberrations. Furthermore, acoustical imaging systems known to the prior art were troubled by the reverberations of ultrasonic waves within the various media located between the transducers and the lens focus. The water bath between the array of transducers and the ultrasonic lens was a particular source of such reverberations.